1. Field of the Invention
The present invention relates generally to rotary bits for drilling subterranean formations, and more particularly to a method for fabricating such bits and components thereof by the directed, controlled deposition and affixation of materials used to form the bit. The method disclosed dispenses with the use of molds or preliminary artwork to define the profile of the bit and the topography of the bit face, as well as the need for at least some, if not all, of the precast or otherwise prefabricated components normally employed to define internal elements within the bit matrix.
The method may be employed to fabricate an entire bit body, or bit body components which may be subsequently assembled with other components fabricated by the same method or otherwise. The assembled components are then secured together to form the bit body.
2. State of the Art
Earth boring drill bits employing fixed cutting elements on the bit face, commonly termed rotary drag bits or simply "drag bits", include a bit body formed of steel or a matrix of hard particulate material such as tungsten carbide infiltrated with a binder, generally of copper alloy. The bit body is secured to a steel shank having a threaded pin connection for securing the bit to the drive shaft of a downhole motor or directly to drill collars at the distal end of a drill string rotated at the surface by a rotary table or top drive.
In the case of steel body bits, the bit body is usually machined from round stock to the shape desired, usually with internal watercourses for delivery of drilling fluid to the bit face, and topographical features are then defined at precise locations on the bit face by machining, typically using a computer-controlled five-axis machine tool. Hard facing is applied to the bit face and to other critical areas of the bit exterior, and cutting elements are secured to the bit face, generally by inserting the proximal ends of studs on which the cutting elements are mounted into apertures bored in the bit face. The end of the bit body opposite the face is then threaded, made up and welded to the bit shank.
In the case of a matrix-type bit body, it is conventional to employ a preformed so-called bit "blank" of steel or other suitable material for internal reinforcement of the bit body matrix. The blank may be merely cylindrical and tubular, or may be fairly complex in configuration and include protrusions corresponding to blades, wings or other features on the bit face. Other preform elements comprised of cast resin-coated sand, or in some instances tungsten carbide particles in a flexible polymeric binder, may also be employed to define internal watercourses and passages for delivery of drilling fluid to the bit face, as well as cutting element sockets, ridges, lands, nozzle displacements, junk slots and other external topographic features of the bit. The blank and other preforms are placed at appropriate locations in the mold used to cast the bit body. The blank is bonded to the matrix upon cooling of the bit body after infiltration of the tungsten carbide with the binder in a furnace, and the other preforms are removed once the matrix has cooled. The threaded shank is then welded to the bit blank. The cutting elements (typically diamond, and most often a synthetic polycrystalline diamond compact or PDC) may be bonded to the bit face during furnacing of the bit body if thermally stable PDC's, commonly termed TSP's are employed, or may be subsequently bonded thereto, as by brazing, adhesive bonding, or mechanical affixation.
As may be readily appreciated from the foregoing description, the process of fabricating a matrix-type drill bit is somewhat costly and is an extremely complex two-step process requiting production of an intermediate product (the mold) before the end product (the bit) can be cast. The blanks and preforms employed must be individually designed and fabricated, and even minor changes in a drill bit design may necessitate the use of new and different preforms. The mold used to cast the matrix body must be machined in a cylindrical graphite element in a very precise manner, particularly as to bit face topography, and the preforms themselves placed at precise locations within the mold to ensure proper placement of cutting elements, nozzles, junk slots, etc.
For many years, bit molds were machined to a general bit profile, and the individual bit face topography defined in reverse in the mold by skilled technicians employing the aforementioned preforms and wielding dental-type drills and other fine sculpting tools. In more recent years, many details may be machined in a mold using a computer-controlled five-axis machine tool. In either case, machining of the bit mold and placement of the preforms is a time-consuming process, still subject, at least to some extent, to human error.
In some cases the mold fabrication process has been made faster and less costly by use of rubber displacements duplicating in fine detail the topography of an entire bit profile and face, which displacements are then used to cast a bit mold of appropriate interior configuration from which to cast a bit. However, such displacements are only useful for "standard" bits which are fixed in design as to size, number and placement of cutting elements and number of nozzles and thus are cost-effective only for high-volume bits, of which there are relatively few. With frequent advances and changes in bit design, preferences of individual customers for whom bits are fabricated, and the general decline in the number of wells being drilled in recent years, high-volume standard bits have become almost nonexistent.
While matrix-type bits may offer significant advantages over steel body bits in terms of abrasion and erosion-resistance, and while recent advances in matrix technology have markedly increased the toughness and ductility of matrix bodies, in many cases the higher cost of a matrix-type bit and the longer time to fabricate same may result in the customer choosing a cheaper steel body bit with a faster delivery time. State of the art matrix bit fabrication technology has to date been largely unable to overcome the aforementioned advantages of steel body bits and to eliminate much of the complexity in matrix bit fabrication.